Lastly, we have discussed various methods to determine the water content of a soil sample. In this discussion, one will learn about the methods to determine the specific gravity of a soil sample as well as the measurement of the unit weight of a soil sample.
Specific Gravity Measurement (G)
The specific gravity of a soil sample is determined with the help of pycnometer. The method is the same as that of the water content determination, however, for determining the specific gravity, a completely dry soil sample is taken instead of a moist sample.
Specific gravity is generally measured at 27°C. If the specific gravity is required at any other temperature than 27°C then it can be obtained using the formula as given below.
Measurement of Unit Weight
The bulk unit weight of a sample is defined as the total weight per unit volume of the soil mass. It is the same as the bulk density of soil.
For the measurement of the unit weight of a soil sample, weight, as well as volume measurement of the soil mass, is required. The weight of the soil mass can be measured with high accuracy in comparison to the volume of the sample and hence the methods mostly differ in the procedure for the measurement of the volume. The dry unit weight of the soil can be determined once we have determined the bulk unit weight using the formula,
The methods for the determination of the bulk unit weight are discussed below:
Core Cutter Method
This method is mainly used in the filed. In this method, a core cutter of a known volume of 1000 cubic cm is punched into the ground and then, the core containing the soil is taken out. The weight of the core with the soil as well as that of the empty core is measured. The water content determination of the extracted soil sample is also done. The bulk unit weight can be calculated using the relation,
This method is suitable for the soft fine-grained soil in which core cutting can be done properly. The method can’t be implemented for gravelly and dry soils. One more limitation of this method is that it can only be implemented at places where the surface of the soil is visible and the cutter can be easily driven into the soil.
Water displacement method
In this method, the volume of the soil specimen is determined by the water displacement method. The soil mass is coated with paraffin wax to avoid the disintegration of the soil mass. The soil mass is trimmed to a regular shape and then dipped into molten wax so that a thin layer of paraffin wax gets coated over the soil mass.
The waxed specimen is then immersed into the water-displacement container. The volume of water that comes out of the outflow tube gives the volume of the soil specimen. The actual volume of soil specimen, however, is less than the volume of the waxed specimen. The volume of wax is determined from the mass of wax peeled off from the specimen after the test and the mass density of wax.
A representative soil sample is taken from the middle of the soil specimen for water content determination. Once the mass, volume and the water content are determined, bulk density and dry density can be calculated.
Sand Replacement Method
This method is applicable to the field and not suitable for laboratory purpose. A small area is excavated and the excavated soil sample is measured precisely. A calibrated cylinder containing sand is placed over the excavated area and the pit is filled with the sand. The volume of the pit is obtained from the calibrated cylinder. The bulk unit weight is calculated. A representative sample is tested for water content and from that, the dry density is found out. This method is suitable for gravelly, sandy and dry soil.
Water Balloon Method
This method is suitable for the field. In this method, the volume of the excavated pit is obtained by covering the hole with a plastic sheet and filling it with water. The weight of the excavated sample is measured and then bulk density is calculated.
Radiation Method
The radiation method is the recent development in determining the density of soil in the field. These methods are quick and reliable for determining the density of soil.
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